READ ME FIRST: What is Custom Design?

(This forum is for blogs hosted by WordPress.com. If your blog never had a URL like myblog.wordpress.com then you should seek help at WordPress.org.)

WordPress.com blogs are hosted for no charge. You cannot add or modify any PHP on WordPress.com, or add Javascript for security reasons.

You can select from a wide assortment of themes for no charge. Some of these themes allow you to customize colors or images while the rest of the style is fixed.

CSS is a technology that adds style to HTML. A CSS stylesheet contains rules that tell a browser how to display a page. Every theme has a stylesheet and you can not edit these on WordPress.com.

CSS stylesheet rules can be modified by rules appearing later in the code. Example:

blockquote { color: red; }
blockquote { color: blue; }

Because the blue rule comes after the red rule, browsers will color blockquotes blue. The Custom Design feature, which is included with the WordPress.com Premium and WordPress.com Business plans, works by allowing you to add CSS rules after the theme's stylesheet.